Surveys of United States home buyers consistently place a fireplace as a first or second most sought-after "extra." The vast majority of American homeowners, and many living in apartments, desire a fireplace. A traditional residential fireplace is essentially a semi-open box having a pair of side walls, a rear wall, an open front face, a floor where wood chunks are placed or on which a grate for wood chunks is located so that the chunks burn in proximity to the floor, and a flue through a roof of the box.
Typically, homeowners operate fireplaces over a wide range of burn rates, with the average being approximately 4.5 kilograms of dried wood per hour. Because of the large amount of air entering the fireplace through the open face thereof, the temperature above the burning wood is too low to ignite and burn the air pollutants. It is, therefore, desirable to provide a steady, reliable ignition source above a wood charge in a fireplace for initiating and maintaining combustion of the pollutants.
A problem with the traditional fireplace is that there are substantial particulates emitted from the fireplace and which escape into the atmosphere through the flue and a chimney. Only the surfaces of the wood chunks can burn, and the entire chunk gets relatively hot, leading to fractional distillation of organic compounds from within the wood. The compounds are released into a combustion chamber formed by the fireplace. If the compounds are not completely burned, they become air pollutants upon discharge from the flue.
There are approximately 80 regions in the United States which fail to attain environmental standards for particulate matter having diameters of less than 10 micrometers. Residential wood combustion, typically from wood burning fireplace sources, is a very substantial reason why these environmental standards are not reached in these regions. The various states of the United States are soon to be required to show significant progress towards achieving attainment of the environmental particulate matter standards.
A current plan for residential fireplaces is to allow only gas log units in new construction and to encourage changeover of existing wood burning fireplaces to gas log units. The gas log units must not be convertible, under the plan, to wood burning fireplaces. Wood burning fireplace use in existing facilities will be banned in the 80 regions on "no burn days."
Because of these plans and the desire of many residents to have wood fireplaces, gas log fireplaces have recently been redesigned to produce flames for closely simulating burning wood flames. These new designs are the result of a recent American Gas Association rule permitting up to 400 parts per million of carbon monoxide in flue gases from gas log fireplaces. However, these substantial carbon monoxide emissions also have a substantial detrimental environmental effect. In addition, it is probable that gas log fireplaces contribute some particulates to the environment.
Hence, there are environmental problems associated with the gas log fireplaces. In addition, most residents prefer an actual wood fire to gas logs, despite the realistic nature of gas log flames.
It is, accordingly, an object of the present invention to provide a new and improved wood burning fireplace and to a method of operating same so there is a substantial reduction of particulate emissions from such fireplaces.
Majestic, the largest United States manufacturer of factory built, zero clearance wood burning fireplaces, recently introduced such a fireplace including a baffle in proximity to the flue, to provide a circuitous flow path for gasses flowing from wood in the fireplace to the flue. The baffle carries secondary air tubes. Air from outside the fireplace is supplied to the secondary air tubes to provide secondary combustion in the fireplace, and thereby, reduce, to a certain extent, particulate emissions flowing from the wood fire to the flue. The typical wood burning fireplace has a particulate emission factor of 10.8 grams per dry kilogram of burned wood. In the Majestic wood burning fireplace, the particulate emission factor is reduced to the 2-4 gram per kilogram range.
In the co-pending application, there is disclosed a woodstove in which gas from a hydrocarbon source outside of the woodstove flows to a pilot source in a secondary combustion zone of the woodstove. A gas flow valve for the hydrocarbon gas from the external source and an ignitor for the pilot are controlled in response to a sensor for the presence and absence of the pilot flame.
It is another object of the invention to provide a new and improved wood burning fireplace and method of operating same, incorporating techniques and apparatus similar to those disclosed in the co-pending application.
Another object of the invention is to provide an insert for a wood burning fireplace for substantially reducing the amount of particulates emitted from the fireplace.
A further object of the invention is to provide a wood burning fireplace and a method of operating same wherein particulate emissions are less than one gram for each kilogram of dry burned wood.